mm, x86: Saving vmcore with non-lazy freeing of vmas

During the reading of /proc/vmcore the kernel is doing
ioremap()/iounmap() repeatedly. And the buildup of un-flushed
vm_area_struct's is causing a great deal of overhead. (rb_next()
is chewing up most of that time).

This solution is to provide function set_iounmap_nonlazy(). It
causes a subsequent call to iounmap() to immediately purge the
vma area (with try_purge_vmap_area_lazy()).

With this patch we have seen the time for writing a 250MB
compressed dump drop from 71 seconds to 44 seconds.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: kexec@lists.infradead.org
Cc: <stable@kernel.org>
LKML-Reference: <E1OwHZ4-0005WK-Tw@eag09.americas.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/crash_dump_64.c

@@ -46,6 +46,7 @@ ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
 	} else
 		memcpy(buf, vaddr + offset, csize);
 
+	set_iounmap_nonlazy();
 	iounmap(vaddr);
 	return csize;
 }